A radio access technology (RAT) is the underlying physical connection method for a radio-based communication network. Well known examples of RATs used by mobile network operators include LTE, UMTS and GSM.
It is not uncommon for a mobile network operator to support more than one RAT concurrently in the same coverage area. Typically, although not necessarily, the support of multiple RATs is necessitated by the need to support legacy communication devices that are not compatible with a more advanced RAT. For example, a UMTS-based communication device may be incompatible with an LTE-based mobile network. Accordingly, even after introducing an LTE-based network, a mobile network operator may continue to operate a UMTS-based network in the same general coverage area in order to support customers that do not own LTE compatible communications devices.
Some communications devices are compatible with multiple RATs. For example, an LTE communication device may be configurable to work with a UMTS-based network. A mobile network operator may therefore use a legacy network, i.e., the UMTS-based network per the previous example, to provide extra capacity for properly configured communication devices in the event that a more advanced network, e.g. an LTE-based network as per the previous example, is unable to provide service for some or all of the communication devices in its coverage area. In such a case, a cell controller in the more advanced network may handoff the communication device to a cell controller associated with the legacy network in the same coverage area.